This invention is in the field of electronic measurement. Embodiments are more specifically directed to devices and methods of measuring the impedance of a circuit element.
As fundamental in the art, the electrical impedance of an electrical circuit or circuit component is the opposition to current that the circuit or component presents to an applied voltage. In general, impedance is a complex quantity, namely the sum of a resistance and a reactance, and varies with the frequency of the applied voltage. Impedance is of course an important factor in the manufacture of electronic circuits and systems, especially in determining the efficiency with which energy is delivered to the load of a circuit. In addition, impedance measurement and analysis can be used in electronic sensors, for example in determining the properties of a material or workpiece, or conditions of the surrounding environment.
Conventional impedance analyzers operate by applying a sinusoidal stimulus to the object under measurement (referred to herein as the “device under test”, or “DUT”), and measuring the electrical response of the DUT to that sinusoid waveform. Typically, the response is measured at more than one frequency of the sinusoidal stimulus, for example over a “sweep” of input frequencies. The use of a single frequency sinusoid as the measurement stimulus at each of the frequencies of interest greatly simplifies the measurements, as harmonic interference in the response of the DUT is largely avoided.
Many modern electronic integrated circuits integrate essentially all necessary functional components of a computer system, whether general purpose or arranged for a particular end application. Those large scale integrated circuits that include the computational capability for controlling and managing a wide range of functions and useful applications are often referred to as a microcontroller, or in some implementations as a “system on a chip”, or “SoC”, device. Typical modern microcontroller architectures include one or more processor cores that carry out the digital computer functions of retrieving executable instructions from memory, performing arithmetic and logical operations on digital data retrieved from memory, and storing the results of those operations in memory. Other digital, analog, mixed-signal, or even RF functions may also be integrated into the same integrated circuit for acquiring and outputting the data processed by the processor cores.
The efficiencies provided by microcontrollers and SoCs have reduced the cost of implementing complex measurement and computational functions in applications for which such functionality had been cost-prohibitive. For example, sensors and controllers are now being deployed in a wide range of applications and environments, including in the widely-distributed networks of such sensors and controllers often referred to as the “Internet of Things” (IoT).
For these reasons, microcontroller-based sensors for the measurement and analysis of electrical impedance is attractive. FIG. 1 illustrates a conventional microcontroller-based impedance analyzer. In this example, microcontroller 10 includes digital frequency synthesizer 2, which generates a sample stream corresponding to the desired signal waveform indicated by signals from processor 5. In this example, this sample stream corresponds to a sinusoidal waveform of a selected frequency. The sample stream generated by digital frequency synthesizer 2 is applied to digital-to-analog converter (DAC) 4, which is also realized within microcontroller 10, and which generates the output sinusoidal stimulus Vin that will be applied to the device under test (DUT) 15 for measurement of its impedance. DUT 15 is a two-terminal device, having one terminal receiving stimulus voltage Vin (after additional filtering, if desired), and its other terminal coupled to the inverting input of operational amplifier 16. Op amp 16 receives a reference voltage, for example at ½ the peak-to-peak amplitude of stimulus voltage Vin, at its non-inverting input. Reference impedance 18 is connected in negative feedback fashion between the output of op amp 16 and its inverting input. The output voltage Vmeas from op amp 16 is received by microcontroller 10, and converted to the digital domain by analog-to-digital converter (ADC) 6.
In this conventional inverting amplifier arrangement, the ratio of output voltage Vmeas to stimulus voltage Vin reflects the impedance of DUT 15 relative to the impedance ZREF of reference impedance 18. Op amp 16 maintains a virtual ground at its inverting input, and as such the voltage drop across DUT 15 will be the input voltage Vin. Additionally, because the input of op amp 16 exhibits a significantly higher impedance than feedback impedance ZREF, effectively all of the current conducted through DUT 15 will pass through feedback impedance ZREF. Output voltage Vmeas will thus be proportional to this DUT current conducted through feedback impedance ZREF. For example, if the impedance of DUT 15 exactly matches the feedback impedance ZREF, output voltage Vmeas will match stimulus voltage Vin. Accordingly, the impedance of DUT 15 can be determined from the output voltage Vmeas presented by op amp 16. As mentioned above, this measurement is performed over frequency by the conventional architecture of FIG. 1, typically by processor 5 controlling digital frequency synthesizer 2 to sweep the frequency of the stimulus voltage Vin applied to DUT 15. ADC 6 samples and digitizes output voltage Vmeas representing the response of DUT 15 to the stimulus at each frequency, and processor 5 analyzes that sample stream, for example via a discrete Fourier transform (DFT), to determine the impedance of DUT 15 at each frequency in the sweep. Both the amplitude and phase of output voltage Vmeas relative to stimulus voltage Vin are considered in quantifying the inductive and capacitive components of the impedance of DUT 15.
As shown in the conventional arrangement of FIG. 1, DUT 15 is connected in parallel with calibration impedance 14, with switches 13 selecting one or the other of these loads. As known in the art, calibration impedance 14 is a known precision impedance that is useful in calibrating the impedance measurement for non-idealities in op amp 16 or presented by the text fixture retaining DUT 15. As suggested in FIG. 1, calibration impedance 14 and may be a variable impedance device (e.g., a bank of selectable precision resistors) to provide accurate calibration over a wide range of impedances. Similarly, reference impedance 18 may also be a variable impedance so as to better match the expected impedance of DUT 15.
While this conventional architecture is capable of analyzing a wide range of load impedances, the use of a sinusoidal stimulus voltage Vin requires the relatively costly circuitry of digital frequency synthesis function 2 and DAC 4, especially if impedance is to be measured at reasonably high precision and at fine resolution. In particular, the number of bits of resolution in the sample stream of the stimulus waveform, as well as the sample rate of that sample stream, translates directly into the complexity of the DAC circuit. As is well known in the art, complex DAC circuits consume significant chip area, and can significantly increase the cost of the microcontroller device. This cost factor can be significant in modern embedded processors and SoC devices, and can limit the sensor applications for which impedance measurements can be performed.